1. Field of the Invention
The present invention generally relates to incinerators and more particularly to a method and apparatus for the active control of compact waste incinerators. Specifically, the present invention provides a means to improve the combustion efficiencies of various combustion devices such as incinerators through the production, placement and stabilization of large scale vortices within a combustion chamber coupled with the controlled and synchronized injection of the fuel and waste relative to the large scale vortices.
The present invention further provides an alternative means to control the formation of soot or other emissions that result from standard combustion processes. The production or reduction of soot is controlled by virtue of synchronizing the fuel injection with the intentional formation of large scale vortices proximate the entrance to the combustion chamber.
2. Related Art
The destruction of hazardous waste in an incinerator is dominated by several factors including the combustion temperatures, fuel and waste residence time, and the fine-scale mixing of the fuel, waste, and oxidizer. Existing incineration technology includes rotary kilns, fixed and multiple hearth incineration devices, fluidized bed incinerators, and liquid injection incinerators. These incineration devices are typically large incinerators which rely on high heat capacity and long residence time to achieve the required destruction capacity demanded of most incinerators. These devices achieve the long residence times and high heat capacity by typically using a very large combustion chamber. Consequently the total operational cost of these conventional incineration devices is relatively high and unacceptable emissions often result when the incinerator is operated outside the optimum or design conditions.
Compact incinerators, derived from commonly used aerospace propulsion technology, such as a ramjet, can achieve the required destruction capacity in a small-scale device, with much shorter incineration time and higher combustion efficiency. Compact incinerators have the additional advantage of eliminating the need to transport waste from remote locations to a central incineration facility by allowing on-site disposal using a small scale device at a reasonable cost.
Compact incineration devices, however, require a highly optimized and effectively controlled combustion process to achieve and maintain reliable operation. Significant areas of concern for compact incineration devices include achieving increased thermal destruction efficiency, yet maintaining pollutant emission control. Increased thermal destruction efficiency pertains to the complete destruction of various types of waste, including hazardous waste, using an incineration process. A certain level of thermal destruction efficiency is needed to certify an incinerator for industrial use. The incineration process as well as other combustion or chemical processes, also need to be accomplished with minimal amount of particle emissions and gaseous pollutant emissions.
Consequently, there exists a continuing need for a reliable and relatively inexpensive method of actively controlling any combustion process in order to improve the overall performance of the combustor. More specifically there is a need to provide a means to control an incineration or combustion process in order to obtain high efficiencies with low pollutant emissions. Higher efficiencies and lower emissions of an incinerator will allow more latitude in the design of compact incinerators.
In addition, there is also a need to develop an alternative means to actively control the soot emissions in a given combustion process without adjusting the quantity of fuel required. In many combustion systems, it is desired to minimize the formation of soot in order to increase the efficiency. However, for energy exchange systems, the high radiation of soot is sometimes beneficial, and therefore the production of soot is encouraged. Historically, the production or reduction of soot was controlled by adjusting the quantity of fuel introduced into the combustion process. This approach however, had several serious disadvantages including a high cost of fuel required to produce a given amount of soot, or inability to maintain the combustion process due to insufficient fuel.
Passive and active combustion control has been applied to ramjets in order to improve the performance of an enclosed combustor. This control, which is based on detailed understanding of the interactions between shear flow dynamics, combustion, and acoustics, resulted in increased heat release, wider flammability limits, and reduced pressure oscillations.
The combustion characteristics of an enclosed combustor, including flammability limits, instability, and efficiency is closely related to the interaction between shear flow dynamics of the fuel and air flow at the inlet and acoustic modes of the combustor. Strong interaction, between the acoustic modes of the combustor and the airflow dynamics may lead to highly unstable combustion. Specifically, unstable combustion may occur when the acoustic modes of the combustor match the instability modes of the incoming airflow. For such conditions, the shedding of the airflow vortices upstream of the combustion chamber tends to excite acoustic resonances in the combustion chamber, which subsequently cause the shedding of more coherent energetic vortices. The continued presence of such vortices provides a substantial contribution to the overall efficiency of the combustion process.
Passive control has historically involved modification to the fuel injection distribution pattern and changes to the combustor geometry. For example, in the dump combustor, nonstandard inlet duct cross-sections were used to control the generation and breakdown of large-scale vortices which play a critical role in driving pressure oscillations and determining the flammability limits. Also, passive control of the combustion characteristics has been achieved by utilizing bluff-body flame holders at the downstream facing step into a dump combustor.
In recent years, however, active combustion control has received increasing attention. In active control, various control devices such as actuators are used to modify the pressure field in the system and regulate the air or fuel supply. Several different types of active control devices have previously been used in laboratory experiments. These active control devices include; loudspeakers to modify the pressure field of the system or to obtain fuel flow regulation; pulsed gas jets aligned across a rearward facing step; adjustable inlets for time-variant change of the inlet area of a combustor; and solenoid-type fuel injectors for controlled unsteady addition of secondary fuel into the main combustion zone. These active control devices have proven to be somewhat successful in suppressing pressure oscillations and extending flammability limits when the combustor operates at low heat release rates.
Unfortunately the existing trend in active control techniques for a combustor is towards decreasing performance of the controller with increasing energy levels within the combustor. Reversing this trend is a problem that has received increasing attention.